استراتژی جدید کنترل ارتقا پایداری برای سیستم های شبکه فوتوولتاییک متمرکز در کاربردهای شبکه های هوشمند Novel Stability Enhancing Control Strategy for Centralized PV-Grid Systems for Smart Grid Applications

I. INTRODUCTION THE SUN IS THE ultimate source of energy and it is widely felt that if solar energy is prudently tapped using solar photovoltaic (PV) technology, it can meet all the energy requirements of the mankind. Both crystalline and thin film PV technologies have evolved at tremendous pace, rendering solar PV amongst the most popular alternatives to fossil fuel based conventional energy. Extensive research efforts in the area have led to the possibilities of integrating large, centralized PV generation (CPVG) with the power grid. This, coupled with considerable reduction in PV panel prices, has raised unprecedented interest in this area. CPVG has a huge potential to serve as a large capacity electric energy source [1]. Many countries have already embarked upon ambitious missions of integrating large capacity CPVG with their power grid. In fact, the CPVG can be assigned additional (ancillary) functionalities as part of smart grid structure [2]. The accumulation of a large number of PV-grid systems, especially the CPVG’s, have led to new challenges and issues such as power system stability, grid voltage profile and regulation and power quality. These factors impose an upper limit on the PV penetration for a given capacity feeder or small capacity system. This limit is to guarantee that, the operating parameters are within rated specifications, even under dynamical disturbances to secure both utility and PV distributed generation system (PV-DGS) [3]. This paper focuses on the stability aspect of the power grid under large PV penetration. Talking about stability, a major issue arises on account of the “inertia-less” nature of the power inverters which are used to interface the solar PV with the grid. Although competitive interest for integration of large capacity photovoltaic sources to the grid has resulted in several highly efficient versions of inverters and control schemes—the inertia less interfacing of VSI based PV-DGS continues to impose many limitations on the use of conventional form of inverter control. Conventional power system stabilizers (e.g., governor, AVR etc.) associated with generating stations continue to be the mainstay of a power system’s dynamic stability [4]. However, with advancement in technology, FACTS devices (eg. SSSC, SVC, STATCOM, etc.) are now commonly used for further enhancing the dynamic stability of the power system [5]–[۸]. They are used to damp out the low frequency electromechanical oscillations arising due to disturbances in weak systems. Yet, a wide deviation of power angle and prolonged oscillations may result in the system falling out of step or getting bifurcated into small islands. In the smart grid scenario, this is one of the challenging domains where inertia-free renewable energy sources are coupled to the grid, e.g., high penetration PV systems [9], [10]. Adverse effects of high penetration of large capacity PV sources on the dynamic stability of power system have been studied by some researchers [10]–[۱۲]. Yazdani et al. [13] have presented the modeling procedure of a large PV system to analyze and identify the potential issues with high PV penetration. Du et al. [11] have reported that conventional control of PV generating stations, exceeding a critical penetration limit, induces negative damping torque. Other authors [12] have presented case studies of large PV sources penetrating the conventional power system and have concluded that the power system’s ability to handle electromechanical oscillations reduces with increasing penetration levels. Working on similar lines, some other authors [10] have concluded that 20% PV penetration causes large oscillations of relative power angles between generators followed by fault initiated transients. Other challenging issues associated with large PV penetration, such as voltage variations of load buses and reverse power flow due to the intermittent nature of solar energy, have also been reported by some authors [14]–[۱۶]. They have concluded that the prevailing standards, e.g., IEEE 1547 [3] need appropriate amendments to accommodate advancements in control techniques of the conventional PV-grid system. Some researchers [17] have determined the fault current levels in the presence of large grid tied PV inverters (PV-DGS) while others [1] have discussed the planning of generation, scheduling and power dispatch in the presence of large centralized PV-grid systems. Limited literature is available that discusses the ancillary functionality of DGS for dynamic stability enhancement. Almost all the reported work is based on active power control that requires reliable sources, e.g., fuel cell [18]. Some other authors [2], [19] have recommended modernization of the power grid in the environment of FACTS and DGS by assigning additional functionalities to them for power system security. Overall it is observed that the existing literature mainly discusses the consequences of high PV power injection into the power system and failure of the system parameters under these conditions. To the best of authors’ knowledge, there are not many suggestions or proposals on alternate, better control schemes that would allow higher PV power injection into the existing system without any adverse effects. Moreover, the proposals for enhancing the PV station capacity to integrate with the grid require heavy modification of the infrastructure and integration of energy storage. The presented work focuses on how to improve the dynamic stability of the system in the presence of large capacity PV-DGS. This is achieved by using a special control law that is analytically derived from the structure preserving energy function (SPEF) model. The designed controller works with only locally measurable signals. Hence, it is easy to acquire and integrate them into the system. In addition, the proposed technique is independent of the location of the PV source installation in the system and does not disturb active power control including MPPT. In view of the increasing demand to revise the existing standards, it is fair to assume that in the imminent future dominated by smart grid, it will be possible to operate large PV installations by suitably controlling the critical power system parameters, at least during the transient phase.